Some people in your group need to step up their game. I'm a i105 Dragoon with Liberator and I average somewhere between 370-380 in that fight. I also take towers. We currently run 2 bards in our group and our highest geared one, averages 350-360 in that fight and that's with singing too. Our scholar does a little dps at the start too and can sometimes do up to 220. We solo tank or with a Warrior and he does about 200 dps as well.
Sometimes it has something to do with familiarity as well. Once your group gets comfortable with the fight and mechanics, your overall dps might increase.